The AI in L4D2 seems to be about the same as the AI in 1, and that's just the problem. It hasn't been updated at all for the new tactics that 2 wants you to learn so badly. Closing the distance to melee a smoker tongue off you is fine, but it takes a little more than that to get a jockey off.
One time in Swamp Fever I was smoker'd and tied up around a corner, choking to death. All three AIs stood there, looking in my general direction, but not doing anything. I went from 100 HP to 300 "downed" HP, to dead. I guess they couldn't see where the trouble was (wall in the way), but still - if human players get that giant note that says "Hey, dumbass, your friend's in trouble", I figure the computer should know better than that.